Device provided with a cathode-ray tube protection circuit for crt activated by power switch



Oct. 5, 1965 J. DE GIER 3,210,596

DEVICE PROVIDED WITH A CATHODERAY TUBE PROTECTION CIRCUIT FOR CRTACTIVATED BY POWER SWITCH Filed Oct. '5, 1962 INVENTOR JOHANNES DE GIERY gawk IZAGEN% United States Patent 3,210,596 DEVICE PROVIDED WITH ACATHODE-RAY TUBE PROTECTION CIRCUIT FGR CRT AC- TIVATED BY POWER SWITCHJohannes de Gier, Emmasingel, Eindhoven, Netherlands, assignor to NorthAmerican Philips Company Inc., New York, N.Y., a corporation of DelawareFiled Oct. 5, 1962, Ser. No. 228,546 Claims priority, applicationNetherlands Oct. 9, 1961, 270,069 Claims. (Cl. 315) This inventionrelates to cathode ray tube devices and more particularly to atelevision receiver including means for preventing damage to thesensitive viewing screen when the receiver is turned ofl or in the eventof a power failure.

The invention relates to a device provided with a cathode-ray tubecomprising an electron gun which at least consists of a cathode, acontrol electrode, a first anode and a second anode. The inventionfurther relates to a circuit arrangement for energizing the first anode.

Known devices often have a limitation in that burningin of thefluorescent screen occurs, for example, when the device is switched off.This is particularly true in the case of devices for reproducingtelevision images.

The electrode to which the fluorescent screen of the cathode-ray tube isconnected usually consists of a conductivelayer provided on the innerside of the bulb. This electrode generally has a comparatively highcapacitance with respect to ground, and consequently a considerablecharge, since this electrode is connected to a very high voltage. Afterthe device has been switched off, this high voltage charge may beretained for a comparatively long time.

If, after the device has been switched oif, the voltage of the firstanode decreases rapidly, the cathode ray is usually suppressed only inpart. Consequently, part of the electrons still emanating from theglowing cathode may flow to the fluorescent screen then still stronglypositive, and impinge thereon with a high velocity, as a result of whicha burn may occur, since the electric charge on the screen and theelectrodes connected therewith are neutralized only slowly. If, however,the ray could pass at its full strength, the screen could be dischargedbefore the deflection voltages have disappeared completely.

According to a known solution, it has been suggested to disconnect thefirst anode from the supply source simultaneously with the interruptionof the A.C. supply. Since the first anode is connected to acomparatively large capacitor to eliminate the consequences of anyflash-over from an electrode applied to a high voltage, the positivevoltage of this anode is maintained for a comparatively long time afterthe interruption, so that the ray can pass at its full strength untilthe screen has been discharged.

However, this solution is not effective when the device is switched offby pulling the plug out of the socket, or because of the disappearanceof the supply voltage.

According to the invention, the burning-in of the screen after switchingoff such a device is avoided by providing a first anode which consistsof two parts to which approximately equal potentials are supplied bymeans of switching circuits having different discharge times in a mannersuch that if the potential of one of the anode parts drops below that ofthe other part, the cathode ray no longer impinges upon the screen. Itis then possible to suppress the ray completely, but the ray ispreferably deflected so that it strikes the second anode and dischargesit. Since the second anode is also connected to the electrode which isprovided on the inner wall of the bulb, this electrode, and consequentlyalso the screen, are discharged. In the case where the ray is completelysuppressed, these electrodes remain charged, which may be unpleasantupon touching.

The invention will now be described more fully with reference to theaccompanying drawing in which:

FIG. 1 shows diagrammatically a circuit arrangement of a device inaccordance with the invention, and

FIGS. 2 and 3 show in greater detail two embodiments in accordance withthe invention.

Referring now to FIG. 1, reference numeral 1 denotes a cathode-ray tubecomprising a fluorescent screen 2 and an electron gun consisting of acathode 3 heated by a heat ing member 4, a control electrode 5 withwhich the intensity of the cathode ray can be controlled, a first anode6 consisting of two parts, a second anode 7, 8 and a focusing electrode9. The parts 7 and 8 of the second anode are electrically connectedtogether in the tube and the part 8 is connected electrically 'by meansof a spring 10 to the conductive coating 11 on the inner wall of thetube 1. The electrodes 7, *8 and 11 have a capacitance with respect toground which is represented diagrammatically by the capacitor 12. Theelectrode 11 comes also into contact with the fluorescent screen 2.Through a resistor 13, the electrode 11 is connected to a source of highvoltage 14 (16 kv.) which derives its voltage from the horizontaldeflection circuit. High voltage source 14 is connected at 15 to thesynchronisation separating circuit (not shown) and further to thepositive terminal of the power supply apparatus 16. Deflection coils 37are arranged around the neck of the cathode-ray tube in normal manner.

The efficiency diode circuit 17 of the deflection circuit 14 suppliesthe positive voltage for the parts of the first anode 6 and thefocussing electrode 9 through potentiometers 18 and 19, respectively.The two parts of the first anode 6 are connected to the circuitarrangement 20, two embodiments of which are shown in FIGS. 2 and 3. Inthis case, the control electrode 5 is connected to a video amplifier 21and the cathode 3 to the brightness control consisting of resistors 22,23, 24, 25 and a capacitor 26. Alternatively, the control voltages mayalso be supplied to the cathode 3, in which case the electrode 5 isconnected to the brightness control. A switch 38 controls the supply ofpower to the system.

As shown in FIG. 2, the first anode 6 is divided into two parts by adiagonal cut, so that parts 6a and 6b are formed, which are separatedalong a plane which is at a sharp angle to the axis of the parts. Thepart 6a of the first anode is connected to the capacitor 27 and itspotential is supplied through resistors 28, 29 which constitute avoltage divider. The part 612 of the first anode 6 is connected to apotentiometer 32, the ends of which are connected to resistors 30 and31. The potentiometer 32 renders it possible to make the voltage of thepart 6b exactly equal to that of the part 6a. The electron beam passesin the direction I. Since the circuit branch 30, 31, 32 does not includea capacitor, its R.C. time constant is very short. If the device isswitched oif, the voltage of 6b will decrease much more rapidly thanthat of 6a, as a result of which due to the diagonal cut, the part 6bcauses a deflection of the ray in the direction II, so that this rayimpinges on the second anode 7 and discharges the electrodes 7, 8 and 11before the deflection due to the voltage difierence between electrodes6a and 6b has decreased to zero.

The circuit arrangement may also be simplified, as shown in FIG. 3, inwhich at the same time another embodiment of the first anode isillustrated. The part 60 is then connected to a capacitor 33 and througha resistor 34 of high value to the anode part 6d and the common point ofa voltage divider constituted by resistors 35 and 36.

The voltages of the parts 60 and 60! will then always become equal.After the receiver is switched off, the voltage of 6d will decrease morerapidly than that of 60. Since 6d has a small aperture through which theray passes the control effect of 6d is so great that the ray issuppressed completely. As has been indicated already, the disadvantageof this arrangement is that the electrodes 7, 8 and 11 remain at a highvoltage, which may be unpleasant if the device is opened, for example,for checking, adjusting or repairing purposes. The embodiment of thefirst anode of FIG. 2 is consequently preferred. It should be noted thatthe circuit arrangement of FIG. 3 may also be used with the constructionof the anode parts 6a and 6b shown in FIG. 2, and conversely.

The invention is of particular importance for tubes without an ion trap,and especially if the deflection voltages are produced by means oftransistors, since in this case, after the device has been switched off,they drop to zero substantially immediately, because there are no tubeswith afterglowing cathodes present.

The invention will now be described more fully with reference to anexample.

The voltage of the supply apparatus 16 designated in the drawing by is,for example, 200 v. The high voltage supplied by the circuit arrangement14 is 16 kv. and the voltage supplied by the efliciency diode circuit tothe focusing electrode 9 and the circuit arrangement 20 is approximately800 v.

The resistors and capacitors shown in the drawing may have, for example,the following values.

Resistor 13 MQ 1 Resistor 18 MS2 1.5 Resistor 19 "M9" 1.5

Resistor 22 "9 47,000 Resistor 23 Q 2,200 Resistor 24 Q 6,000 Resistor25 "52.. 21,300

Resistors 28, 29, 30, 31, each Mo 4.7 Resistor 32 MQ 1 Resistor 34 MQResistors 35, 36, each Mt2 2.7

Capacitor 12 pF 1,500 Capacitor 26 pF 80,000 Capacitors 27, 33, each pF68,000

Although only two embodiments of circuit arrangements and constructionsof the first anode have been described, it is obvious that variouschanges and modifications of the invention may be made without departingfrom the scope of the invention.

What is claimed is:

1. A cathode ray tube arrangement comprising a cathode, a first anodecomprising first and second sections insulated from one another, asecond anode, a source of anode voltage, first and second circuit meansfor coupling said voltage source to said first and second sections,respectively, said first and second circuit means having diflerentdischarge time constants, and means for eflectively removing saidvoltage source from said first and second sections thereby to diminishthe anode voltages of said first and second sections in accordance withthe said respective time constants of said first and second circuitmeans.

2. A cathode ray tube arrangement comprising a fluorescent screen, anelectron gun for directing an electron beam to said screen, saidelectron gun comprising a cathode, a first anode and a second anode,said first anode comprising first and second sections insulated from oneanother, a source of anode voltage, first and second circuit means forcoupling said voltage source to said first and second sections,respectively, to provide approximately equal potentials thereto, saidfirst and second circuit means having different discharge timeconstants, and means for effectively removing said voltage source fromsaid first and second sections thereby to diminish the anode voltages ofsaid first and second sections in accordance with the said respectivetime constants of said first and second circuit means, said first andsecond sections being arranged within said tube so as to deflect theelectron beam away from said screen at a given inequality of the anodepotentials of said first and second sections.

3. Apparatus as described in claim 2 wherein each of said first andsecond sections comprises a truncated right circular cylinderlongitudinally displaced from one another along the axis of said cathoderay tube so as to produce a deflection field during the decay of saidanode potentials having a transverse component which directs said beamto said second anode.

4. A cathode ray tube arrangement comprising a cathode, a first anodecomprising first and second spaced apart truncated cylindrical sections,each of said sections having a matching truncated surface separated fromeach other by a diagonal cut along a plane which forms an acute anglewith the longitudinal axis of said cylindrical sections, a second anode,a source of anode voltage, first and second circuit means for couplingsaid voltage source to said first and second sections, respectively,said first and second circuit means having different discharge timeconstants, and means for deenergizing said voltage source thereby todiminish the anode voltages of said first and second sections inaccordance with the said respective time constants of said first andsecond circuit means.

5. A cathode ray tube arrangement comprising a fluorescent screen, anelectron gun for directing an electron beam to said screen, saidelectron gun comprising a cathode, a first anode and a second anode,said first anode comprising first and second spaced truncatedcylindrical sections separated from one another by a plane which formsan acute angle with the longitudinal axes of said cylindrical sections,said first and second sections being electrically insulated from oneanother, a source of anode voltage, first circuit means comprising aresistance-capacitance network for coupling said first section to saidvoltage source, second circuit means coupling said second section tosaid voltage source, said first circuit means having a longer dischargetime constant than said second circuit means, said first and secondcircuit means being arranged to supply approximately equal anodepotentials to said first and second sections, and means for eifectivelydecoupling said voltage source from said first and second sectionsthereby to diminish the anode voltage of said first and second sectionsin accordance with the said respective time constants of said first andsecond circuit means whereby said electron beam is deflected away fromsaid screen and onto said second anode.

6. A cathode, ray tube arrangement comprising a cathode a first anodecomprising a cylinder longitudinally positioned within said tube anddivided into two sections along its longitudinal axis, a second anode, asource of anode voltage, first circuit means for coupling said voltagesource to one of said cylindrical sections, second circuit means forcoupling said voltage source to the other one of said two sections, andfirst and second circuit means having different discharge timeconstants, and means for deenergizing said voltage source thereby toinitiate a decay in the anode voltages of said one and said othercylindrical sections in accordance with the said respective timeconstants of said first and second circuit means.

7. Apparatus as described in claim 6 wherein said first anode cylinderis divided into two separate sections each of which comprises atruncated right circular cylinder, said two sections being electricallyinsulated from one another and said first and second circuit means beingarranged to supply approximately equal potentials to said first andsecond sections.

8. A cathode ray tube arrangement comprising a fluorescent screen, anelectron gun for directing an electron beam to said screen, saidelectron gun comprising a cathode, a first anode and a second anode,said first anode comprising first and second spaced sections insulatedfrom one another, said first section comprising a substantiallydisc-shaped member having an aperture for passage therethrough of saidelectron beam and said second section comprising a cylindrical member, asource of anode voltage, first and second circuit means for couplingsaid voltage source to said first and second sections, respectively,said first and second circuit means having different discharge timeconstants, means for effectively removing said voltage source from saidfirst and second sections thereby to diminish the anode voltage of saidfirst and second sections in accordance with the said respective timeconstants of said first and second circuit means.

9. Apparatus as claimed in claim 8 wherein said second circuit meanscomprises a resistance-capacitance network having a substantially longerdischarge time constant than said first circuit means.

10. A cathode ray tube arrangement comprising a fluorescent screen, anelectron gun for directing an electron beam to said screen along a givenpath, said electron gun comprising a cathode, a first anode and a secondanode, said first anode comprising first and second longitudinallyspaced sections insulated from one another, a source of anode voltage,first circuit means for coupling said voltage source to said firstsection to supply a given potential thereto, second circuit means forcoupling said voltage source to said second section to supply apotential thereto having a predetermined relationship to the potentialof said first section thereby to direct said electron beam to saidscreen, and means for effectively removing said voltage source from saidfirst and second sections, said first and second circuit means havingdifferent discharge time constants whereby a deflection field isproduced by the decay in anode potential of said first and secondsections, said field having a component transverse to said given pathupon a given variation in said predetermined anode potentialrelationship whereby said electron beam is deflected away from saidscreen.

References Cited by the Examiner UNITED STATES PATENTS 2,096,987 10/37Von Ardenne 315-1 2,131,203 3/38 Von Ardenne 31520 2,562,242 7/51 Pohle3l382.1

DAVID G. REDINBAUGH, Primary Examiner.

GEORGE N. WESTBY, Examiner.

1. A CATHODE RAY TUBE ARRANGEMENT COMPRISING A CATHODE, A FIRST ANODECOMPRISING FIRST AND SECOND SECTIONS INSULATED FROM ONE ANOTHER, ASECOND ANODE, A SOURCE OF ANODE VOLTAGE, FIRST AND SECOND CIRCUIT MEANSFOR COUPLING SAID VOLTAGE SOURCE TO SAID FIRST AND SECOND SECTIONS,RESPECTIVELY, SAID FIRST AND SECOND CIRCUIT MEANS HAVING DIFFERENTDISCHARGE TIME CONSTANTS, AND MEANS FOR EFFECTIVELY REMOVING SAIDVOLTAGE SOURCE FROM SAID FIRST AND SECOND SECTION THEREBY TO DIMINISHTHE ANODE VOLTAGES OF